The present invention relates to improvements in an illumination system for illuminating a line on a document. More particularly, the present invention relates to a slot reader or a page reader in which successive lines or segments are illuminated and the images are reflected through a lens to a sensor array.
The illumination systems of the prior art have the limitations that, without modification of the incident light and/or its path, the image at the sensor array (also sometimes referred to as the image plane) is more intense at the center of the array than at the edges. In fact, the light intensity at the sensor array has been theoretically calculated to decrease proportionally to the cosine to the fourth power of the angle between a given incoming light ray and a line through the center of the lens. This means that the illumination at a sensing array is normally highest at the center and reduces progressively toward the edges (as the angle from the center of the lens to a corresponding point on the sensing array increases.) This phenomenon has been, therefore, referred to as a lens-cosine to the fourth power (lens-cosine.sup.4) fall-off.
A similar fall-off of light illumination intensity occurs as a result of use of illumination sources, such as lamps, where designers have traditionally modelled such sources as point sources.
Several techniques have been suggested in previous optical systems for dealing with the lens cosine.sup.4 fall-off phenomenon described above. These techniques include masks, "dog-bones", or gratings which have the effect of removing or discarding part of the incident light in the central regions in an attempt to have a uniform light as sensed at the sensing planar array. These systems described in this paragraph are undesirable because they are inefficient, difficult and expensive to manufacture and pose maintenance difficulties when the lamps need replacing. The efficiency is low because the discarded light means that a greater intensity lamp must be used, creating additional heat and possibly involving an assembly having a greater size. The dog-bones, masks and grating all require additional parts which must be manufactured, stocked and assembled into the system. Further, since these parts are in the vicinity of the lamps, replacement of the lamps may require some disassembly of the masks or gratings, and then require reassembly in the same location after the bulb has been changed.
The prior art has suggested solutions which deal with other problems associated with optical imaging systems including a lens. In U.S. Pat. No. 4,506,152 to Gupta, a system for compensating for lens vignetting is proposed in which a mirror in the form a cylindrical ellipse has end caps and additional reflecting mirrors positioned adjacent the document adjust for the vignetting effect of a lens having a F-number less than 2.0. Lens vignetting refers to a shadow cast by a lens barrel on the lens elements. This patent teaches that lens vignetting of 40% can be reduced to a less than 10% variation in intensity through the use of a five mirror system. Accordingly, this patent addresses a different problem from that discussed with respect to the present invention, and solves its problem by adding a significant number of pieces, each of which adds expense and size and bulkiness to the assembly.
Various other techniques have been proposed to improve the quality of a reflected image, including IBM Technical Disclosure Bulletin, Vol. 15, No. 2, Page 521 directed to reducing the shadows in a photocopy machine.
As is apparent from the foregoing description of the prior art identified above, there are significant disadvantages and limitations in such approaches to providing a quality and uniform image reflected off an input document and onto a planar array sensor.